Members that are joined together by means of the friction stir welding method (FSW) have an almost ideal microstructure structure in the seam region, which to a large extent can be compared with the original material properties of the initial members. By this means the strengths that can be achieved of a joint seam manufactured by means of the friction stir welding method can be almost comparable with those of the solid material. The friction stir welding method is primarily suitable for metals or metal alloys that do not have too high a melting point, such as aluminium, for example. By this means it is possible to form a butt joint between plate-type members with a material thickness of 30 mm and more.
In the friction stir welding process a rotating hard metal welding nib, which has a shoulder in an upper section, is in general introduced into the butt joint, i.e. into what is subsequently the seam formation region between the two members that are to be joined together. The friction heat generated plasticises the metal in the butt joint region and enables firstly the introduction of the rotating welding nib and subsequently its ongoing movement along the joint to form the friction stir welding seam. The shoulder of the welding nib primarily has the task of heating the material by frictional heat, and by this means softening it, while the rotating welding nib stirs the plasticised material in the weld formation region, as a result of which an intimate connection ensues between the members to be joined. The feed of the welding nib requires relatively high forces, so that the members to be joined are arranged on a workpiece support, and must be securely clamped to the latter. In a region underneath the joint, i.e. the seam formation region, extends a strip-type backing, which in the event of wear can easily be replaced, (a so-called easily replaceable backing); this in turn rests against the workpiece support. This backing extends over the whole length of the joint, and moreover slightly beyond the joint at both ends.
The distance of the end of the welding nib from the backing is an important factor in the quality of a friction stir weld seam. If this distance is too small the result can be increased wear and/or total failure of the welding nib as a result of fracture, wherein the backing can also be damaged. If, on the other hand, the distance is too large, the material is not completely plasticised and stirred by the welding nib, with the result that in the rear region, i.e. on the lower face of the weld seam, the members are not fully welded through the thickness. Such a seam in which the weld does not penetrate fully through the thickness represents one or a plurality of so-called root fusion defects (or lack of penetration=LOP); in particular in the event of dynamic loading of the weld seam these are to be viewed as notches that can lead to cracks and thus to a total failure of the member. Deployment of friction stir welded components that have not been tested is therefore impermissible in sectors in which safety is relevant, such as, for example, in aviation or in power station technology.
A major problem consists in being able to detect such root fusion defects reliably, since the lower face of the friction stir weld seam, in particular as a result of the use of the backing, and the metal fused on in the weld formation region, has a very smooth surface and the members, even in the region of the root fusion defects, are still “cold welded”, i.e. for practical purposes are “adhesively” cemented together, or are “interlocked” together, so that in general no cracks occur that can be detected in a straightforward manner. Nevertheless, as a result of the edges of the joint in the region of the root fusion defects that are just “cold welded” together, only a very low mechanical strength is provided in comparison to that of a zero-defect friction stir weld seam; this strength does not even begin to approach the load capacity of the initial members.